1. Field of the Invention
The present invention relates to a polishing method and apparatus for automatic reduction of taper of a wafer in single-wafer polishing, said method and apparatus being suitable for automatically polishing semiconductor wafers one at a time so as to be flat and free of taper through such processes as lapping and mechano-chemical polishing (hereinafter referred to as polishing).
2. Description of the Prior Art
Generally, wafers are obtained by cutting a single crystal rod as a slice. The thus obtained wafers are further processed in a series of processes, such as beveling, lapping, etching, sandblasting, elimination of donors of silicon-oxygen complexes and the like and still further mirror-finished by polishing and then being cleaned to be final products.
As shown in FIGS. 6 and 7, a majority of wafers 9 with a radius R forwarded to the polishing process have a taper 34 and thus the polishing process needs a kind of polishing, in which reduction of taper is carried out at the same time. Moreover, the wafer 9 is not an exactly circular disk, but has an orientation flat 35 (hereafter referred to as OF 35) formed in part of the periphery thereof as also shown in FIG. 6. Conventionally a variety of polishing methods were developed and tested, since taper was difficult to be reduced when wafers were polished only by being pressed to a polishing pad, in particular, under the influence of the taper 34 and OF 35.
On the other hand, there was a conventional multi-wafer polishing method as a method for polishing wafers 9, in which a plurality of wafers 9 are polished at the same time. In the recent trend that requires increase in the diameter and improvement on dimensional accuracy in processing, the single-wafer polishing method has been more popular, in which method wafers are polished one at a time.
A means was adopted so as to be free of taper in the single-wafer polishing method, which is shown in FIG. 8. The means is to bias the center of pressing load from the center of a wafer 9 by a eccentricity of .delta.. It has been not only theoretically but also experimentally proved that the taper 34 is eliminatable by this means.
As shown in FIG. 9, the relation of the eccentricity .delta. and the taper T of a wafer is expressed by a equation EQU .delta.=T.R/8.S.sub.0,
where S.sub.0 is a stock removal by polishing off. The relation between the eccentricity .delta. and the taper T is linear, when the stock removal S.sub.0 is constant and the radius is selected as a parameter.
There is known a disclosure in First Publication of Patent Application HEI No. 2-159722 as one of the prior art in regard to finish by polishing based on the theory above-mentioned. The polishing apparatus for wafers and the positioning device used therefor according to the disclosure have especially such a structure of the device that: wafers as works are held on a X,Y stage; a side of a mounting head, on which wafers to be held by suction is joined with the X,Y stage; the mounting head is positioned relative to the center of the pressing load in such a place that it has a predetermined eccentricity by adjusting a micrometer installed on the X,Y stage, where the position of the mounting head is determined by the displacements thereof in the directions X and Y mutually perpendicular; and the wafers on the X,Y stage are transferred by again being suctioned onto the surface of the mounting head.
Taper of a wafer may be reduced according to the above-mentioned prior art, but the disclosed technology is only fundamental in regard to reducing taper of a wafer in polishing process, for a technology, in which wafers are mounted on a surface of the mounting head with a predetermined eccentricity, is detailed therein without a suggestion on any improvement beyond the fundamentals. The exact positioning of wafers, which serves polishing of high accuracy, may be difficult and time-consuming in the case of multi-polishing according to the disclosure, since the adjustment has to be made manually. More particularly, there remain a problem of poor positioning accuracy, that is, positioning accuracy of the X,Y stage and much of time-consuming steps such as respective determinations of eccentricity of wafers and positioning of the wafers and therefore, in light of the just-mentioned adversaries the prior art has a plurality of problems to be solved before being put to practical use.
The present invention has an object to provide a method and an apparatus for polishing to reduce or eliminate taper of wafers in single-wafer polishing by automation of steps of measurements of taper and polishing of wafers including amendments of conditions for further eliminating taper in corrective polishing. To determine the eccentricity of a wafer according to the prior art, there is a fundamental difficulty, which means that the eccentricity should be computed by the equation EQU .delta.=T.R/8.S.sub.0,
after the taper of a wafer T and a stock removal S.sub.0 is obtained but the eccentricity .delta. is actually difficult to determine, since the thickness profile in the directions X,Y mutually perpendicular is changeable along the respective directions with complexity. The prior art does not practically disclose any means to solve this particular problem.